Adaptive equalizer for high-speed serial data

ABSTRACT

An adaptive equalizer for high-speed serial data comprises a programmable equalizer for equalizing an input serial data signal to generate an equalized serial data signal, wherein the equalization is based on an optimal equalization mode; a signal quality meter for computing an eye width indication based on the equalized serial data signal, wherein the eye width indication is an indicative of the quality of the equalized serial data signal; and a decision unit for determining the optimal equalization mode based on the eye width indication.

TECHNICAL FIELD

This invention generally relates to processing and equalizing high-speedserial data.

BACKGROUND OF THE INVENTION

High-speed interface links connecting a source device to a sink deviceover a physical cable are typically serial communication links. Examplesfor such links include, but are not limited to, a high-definitionmultimedia interface (HDMI), a digital video interface (DVI),DisplayPort (DP), digital interface for video and audio (DiiVA),Universal Serial Bus 3 (USB3), and others. A receiver at the sink devicerecovers data and clock signals transmitted over the cable using a clockdata recovery (CDR) circuit. Typically, the physical cable exhibits thecharacteristics of a low-pass filter. Therefore, the amplitude of therecovered data, received at the receiver, is attenuated and the phase isdistorted. Also, the physical cable typically consists of wires whichare not perfectly shielded. Thus, noise is present in the recovered datadue to cross-coupling between signals from different wires.

Adaptive equalizers are used to restore signal integrity by compensatingfor the frequency dependent attenuation that occurs during transmissionof serial data over the physical cable. However, circuitry forperforming the attenuation estimation needed for adaptive equalizationhas been complex and difficult to implement, specifically whenestimating the attenuation of data transmitted over each of two or morechannels of a multi-channel serial link, in order to perform adaptiveequalization of the data transmitted over each channel.

An equalizer can generally be modeled as a filter. If the cable overwhich a signal is transmitted has a transfer function H(s), where ‘s’ isthe complex frequency, the ideal filter has the inverse transferfunction H⁻¹(s). Additionally, if noise is injected into thetransmission system, the ideal filter should reject such noise as longas the noise is outside the bandwidth of the useful signal. If the noiseis inside the bandwidth of the useful signal, a trade-off can be madeabout the degree of noise rejection together with the useful signalportion. The optimal trade-off would be such that the signal-to-noiseratio is maximized. Thus, the problem of adaptive equalization istwo-fold: estimation of the inverse signal transfer function andestimation of the optimal noise rejection function.

Conventional adaptive equalizers typically examine the eye diagram ofthe input serial data and equalize the input data accordingly. Forexample, US Patent Application Publication No. 2008/0247452 to Lee, etal. (hereinafter Lee) teaches an adaptive equalizer that uses a 2-timesoversampling Bang-Bang phase detector. Such a phase detector recoversthe input data and generates binary timing information (Up/Down)indicating the timing of the edge clock compared to the center clock inthe recovered input data. A data decode block then decodes the Up/Downtiming information and the data pattern and determines whether theUP/Down timing information and the data pattern output by the phasedecoder indicate a need to increase the equalization coefficients of anequalizer core. The equalizer core receives the input data and providesthe equalized data, based on the equalization coefficients set for theequalizer. A bang-bang phase detector has very high jitter. It treatsevery positive phase shift as 180° and every negative phase shift as−180°. On the other hand, a linear phase detector (e.g., a Hogge phasedetector) would measure the phase more accurately and thus providebetter information for choosing the optimal equalizer.

The limitations of conventional adaptive equalizers, such as thatdisclosed in Lee include their limited performance and inability toefficiently equalize high rate serial data. These limitations result, inpart, from the 2-times oversampling phase detector, which leads to abinary correction. Specifically, for high transmission rate, e.g., 3Gbps and above, it is almost impossible to lock on a received signal.Thus, the phase detector in most cases would be out of range.Furthermore, the limitations of conventional adaptive equalizersprohibit the utilization of low-cost and low-quality physical cables forthe multimedia interface. In addition, the length of the physical cableis limited. As with any interconnect, signal attenuation andinterference of such cables increase with cable length and thesignal-to-noise ratio decreases with cable length. Thus, whenimplementing the conventional adaptive equalizer in a receiver of amultimedia interface, low-cost, low-quality, and long distance physicalcables cannot be utilized, as conventional adaptive equalizers areincapable of properly restoring receiver signals.

It would be, therefore, advantageous to provide an improved adaptiveequalizer for high-speed serial links.

SUMMARY OF THE INVENTION

Certain embodiments of the invention include an adaptive equalizer forhigh-speed serial data. The adaptive equalizer comprises a programmableequalizer for equalizing an input serial data signal to generate anequalized serial data signal, wherein the equalization is based on anoptimal equalization mode; a signal quality meter for computing an eyewidth indication based on the equalized serial data signal, wherein theeye width indication is an indicative of the quality of the equalizedserial data signal; and a decision unit for determining the optimalequalization mode based on the eye width indication.

Certain embodiments of the invention further include a method foradaptively equalizing high-speed serial data. The method comprisesequalizing an input serial data signal to generate an equalized serialdata signal, wherein the equalization is based on an optimalequalization mode; generating an eye width indication based on theequalized serial data signal, wherein the eye width indication is anindicative of the quality of the equalized serial data signal; anddetermining the optimal equalization mode based on the eye widthindication.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter that is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other objects, features, andadvantages of the invention will be apparent from the following detaileddescription taken in conjunction with the accompanying drawings.

FIG. 1 is a block diagram of an adaptive equalizer in accordance with anembodiment of the invention;

FIGS. 2A and 2B illustrate eye diagrams of a wide eye and a narrow eyein horizontal and vertical directions.

FIG. 3 is a block diagram of the signal quality measurement unit inaccordance with an embodiment of the invention;

FIG. 4 is a flowchart depicting a decision process performed by thedecision unit in accordance with an embodiment of the invention; and

FIG. 5 illustrates a measured eye diagram and a histogram of the eyeaperture.

DETAILED DESCRIPTION OF THE INVENTION

The embodiments disclosed by the invention are only examples of the manypossible advantageous uses and implementations of the innovativeteachings presented herein. In general, statements made in thespecification of the present application do not necessarily limit any ofthe various claimed inventions. Moreover, some statements may apply tosome inventive features but not to others. In general, unless otherwiseindicated, singular elements may be in plural and vice versa with noloss of generality. In the drawings, like numerals refer to like partsthrough several views.

FIG. 1 shows an exemplary and non-limiting block diagram of an adaptiveequalizer 100 in accordance with an embodiment of the invention. Theadaptive equalizer 100 includes a programmable equalizer 110, a signalquality meter 120, a decision unit 130, and a read-only memory 140. Theprogrammable equalizer 110 adjusts the shape of the input serial datasignals 101 to generate equalized serial data signals 102 for the signalquality meter 120. The programmable equalizer 110 typically implementsan inverse transfer function of the physical cable and includes passiveelectrical components, such as capacitor-inductor or resistor-capacitorcircuits and possibly active electrical components, such as voltagesources or current sources. In accordance with an embodiment of theinvention, in order to support a variety of physical cables, each ofwhich has a different transfer function, the programmable equalizer 110is designed to support a variety of transfer functions. With this aim,the programmable equalizer 110 can be set to different equalizationmodes. Each mode defines a set of parameters defining the selections andvalues of components of the programmable equalizer 110. The parametersassociated with each equalization mode are stored in the memory 140.

The input serial data signals 101 are high-speed signals transmitted ata transfer rate of typically 3 Gbps or more over a physical cableconnecting a source device and a sink device. The input serial datasignal 101 may be compliant with at least one of the followingstandards: a HDMI, DisplayPort, Universal Serial Bus 3 (USB3), PCIe,PCI-x, and the like.

The signal quality meter 120 generates an eye width indication 103 basedon equalized serial data signals 102. The eye width indication is ametric for the quality of the input signal 101. In one embodiment of theinvention, the eye width can be measured in the horizontal direction bysampling the signal 102 in the time domain. In another embodiment, theeye width can be measured in the vertical direction by sampling thesignal 102 in the voltage domain.

In the time, a serial data bit, present during one unit interval (UI) ofa signal 102, is sampled in N phases. If the eye is wide, most of thephases sample the correct value of the data bit. If the eye is narrow,only a few phases in the center of the eye sample the correct value ofthe data bit. For example, FIG. 2A shows eye diagrams 210 and 220presenting respectively a wide eye and a narrow eye for 8 (N=8) phasesamples. The horizontal eye width indication 103 can be defined as thenumber of contiguous phases for which no bit transitions are observed,e.g., phases 0-7 in diagram 210 and phases 2-5 in diagram 220.

FIG. 2B shows eye diagrams 230 and 240 respectively present a wide eyeand a narrow eye measured in the voltage domain. The vertical eye widthindication can be defined as the number of voltage levels reached by thesignal. Counters are used to indicate how often each voltage level isreached. A counter value of zero indicates a certain voltage level isnever reached. Without restriction of generality, the eye width can berepresented as a weighted sum of horizontal and vertical eye width. Inthe case that only horizontal eye width is used, the weighting factorfor vertical eye width is zero. It should be noted that in both cases(vertical and horizontal), the eye width depends on both the signalattenuation and the injected noise, the eye width indication is suitablefor identifying the programmable equalizer 110 with the best signaltransfer function and the best noise transfer function.

The eye width indication 103 is fed to the decision unit 130 which basedon this input determines and sets the programmable equalizer 110 withthe optimal equalization mode. The decision unit 130 executes a processthat analyzes the eye width indication 103 which is the indicative ofthe quality of the equalized serial data signals 102 for the currentequalization mode, and changes the equalization mode of the programmableequalizer until the optimal equalization mode is found. The decisionprocess is described in greater detail with reference to FIG. 4.

FIG. 3 shows an exemplary and non-limiting block diagram of the signalquality meter 120 implemented in accordance with an embodiment of theinvention. The signal quality meter 120 includes a clock and datarecovery (CDR) circuit 320, N transition detectors 330-1 through 330-N,N counters 340-1 through 340-N, N comparators 350-1 through 350-N, apattern analyzer 360, and a timer 370 that times a programmable countingperiod.

The CDR circuit 320 recovers the equalized serial data signals 102 byusing N over-samplers 321-1 through 321-N and an N-phase sampling clock322. The parameter N defines the number of sampled phases and is alwaysgreater than 2. Each sampling signal generated by the clock 322 isshifted in phase by a factor 1/N of the clock cycle with respect to thepreceding signal. The over-samplers 321-1 through 321-N receive anequalized serial data signal 102 that includes N input bits. Using thesampling clock signals, the over-samplers 321-1 through 321-N generate,for each input bit of data, a number N of samples of a unit interval(UI) of data.

The transition detectors 330-1 through 330-N are respectively connectedto N over-samplers 321-1 through 321-N, to detect when rise or falltransitions occur at each phase sample. A rise transition is when a bitvalue is changed from ‘0’ to ‘1’ and a fall transition is when the bitvalue is changed from ‘1’ to ‘0’ at a phase sample. The outputs of thetransition detectors 330-1 through 330-N are respectively fed tocounters 340-1 through 340-N which count the number of transitions. Thecount is performed during a programmable counting period being timed bythe timer 370. At the end of the counting period, the outputs of thecounters 340-1 through 340-N is respectively input to the comparators350-1 through 350-N.

Each comparator 350-1 through 350-N translates its respective output toindicate if the sampled phase is inside or outside the eye. With thisaim, each comparator 350-1 through 350-N compares the number of countedtransitions to a programmable threshold. Counts above the thresholdindicate that the sampled phase is outside the eye and counts below thethreshold indicate the sampled phase is inside the eye. The term “insidethe eye” or “outside the eye” refers to a location of the sampled phaserelative to an eye diagram of an equalized signal.

For example, FIG. 5 shows an eye diagram 500 where the hexagon 510 isthe delimiter area when the eye is open. The histogram 520 depicts phasesamples. At the peak of the histogram 520 (where the histogram's areaoverlaps with the hexagon 510) are phase samples “inside the eye” wherethe eye is most open. The histogram's area that does not overlap withthe hexagon 510 are phases outside the eye. Phase samples inside the eyeare indicative of low number of transitions, hence represent a robustand un-distorted input signal. Phase samples outside the eye areindicative of high number of transitions, hence represent a noisy andattenuated input signal.

In one embodiment of the invention, each of comparators 350-1 through350-N generates a ‘0’ value for phases inside the eye and ‘1’ for phasesoutside the eye. Therefore, the output of the comparators 350-1 through350-N is a pattern that includes N bits.

The pattern analyzer 360 generates the eye width indication 103 byidentifying the longest sequence of contiguous phases inside the eye.The length of this sequence is defined as the eye width indication. Inaccordance with an embodiment of the invention, the pattern analyzer 360identifies the longest sequence of adjacent zeros in the comparators'350-1 to 350-N output pattern as the eye width indication.

As mentioned above the decision unit 130 utilizes the received eye widthindication 103 to determine the equalization mode of the programmableequalizer 110 that would provide the best equalization, thereby thewidest eye width. A non-limiting and exemplary flowchart 400 depictingthe process performed by the decision unit is provided in FIG. 4.

In order to find the optimal equalization mode that would achieve thewidest eye width, a plurality of equalization modes are tested. Theparameters associated with each equalization mode are maintained in thememory 140. The decision unit 130 iteratively sets the programmableequalizer 110 to a different equalization mode. The equalization modethat results the widest eye is set to be the optimal equalization modefor the programmable equalizer 110. The process performed by thedecision unit begins when initializing the channel on which serial datais transmitted.

At S410, parameters of a first equalization mode are retrieved from thememory 140. At S420, the programmable equalizer 110 is set with theretrieved parameters. As a result, an input signal 101 is equalizedaccording to the parameters of the new equalization mode and a new eyewidth indication is computed by the signal quality meter 120. At S430,the computed eye width indication is received at the decision unit 130,which compares, at S440, the received indication to a best eye widthindication. The best eye width indication corresponds to an equalizationmode that yields the best result, i.e., widest eye indication forequalization modes that have been previously tested. For the first test,the best eye width indication is preconfigured with a default value,e.g., the narrowest eye width.

If the new eye width indication shows improved results over the bestwidth indication, then execution continues with S450 where the best eyewidth indication is set to be the new eye width indication, thereby theprogrammable equalizer is set with the optimal equalization mode.Otherwise, execution continues with S460 where it is checked if thereare more equalization modes to test. If not, at S470, parameters of anext equalization mode to be tested are retrieved from the memory 140and execution returns to S420; otherwise, at S480, the equalization modedetermined to be the optimal equalization mode is saved in the memory.This can be used to preset the programmable equalizer 110 with theoptimal equalization mode when the channel is reinitialized.

In accordance with an embodiment of the invention only a predefined setof equalization modes kept in a sorted list are tested. The sorted listis created by pre-characterizing the behavior of the programmableequalizer 110 in a predefined range of equalization parameters. If thecharacteristics result in only one maximum eye width within thecharacterization range, then the equalization modes associated withthese parameters are stored in a sorted list, where the equalizationmodes are sorted according to the progression of the equalizationparameters.

According to this embodiment, the decision unit 130 tries parametersassociated with equalization modes in the sorted list until the maximumeye width is encountered. The eye width is known to have only one globalmaximum as a function of the equalization parameters. The maximum eye isnot known in advance.

To search the optimal equalization mode in the sorted list, the decisionprocess can first try the equalization mode at one end of the sortedlist and progressively check other equalization modes until finding themaximum eye width. Alternatively, the decision process may start thesearch with an equalization mode located in the middle of the sortedlist and try other equalization modes in either direction from themiddle of the list. Then a decision is taken toward direction the eyewidth improves and then progresses in that direction until a maximum eyewidth is encountered.

The principles of the invention may be implemented as any combination ofhardware, firmware, and software. Moreover, the software is preferablyimplemented as an application program tangibly embodied on a programstorage unit or computer readable medium. One of ordinary skill in theart would recognize that a “machine readable medium” or computerreadable medium is a non-transitory medium capable of storing data andcan be in a form of a digital circuit, an analogy circuit, a magneticmedia or combination thereof. The application program may be uploadedto, and executed by, a machine comprising any suitable architecture.Preferably, the machine is implemented on a computer platform havinghardware such as one or more central processing units (“CPUs”), amemory, and input/output interfaces. The computer platform may alsoinclude an operating system and microinstruction code. The variousprocesses and functions described herein may be either part of themicroinstruction code or part of the application program, or anycombination thereof, which may be executed by a CPU, whether or not suchcomputer or processor is explicitly shown. In addition, various otherperipheral units may be connected to the computer platform such as anadditional data storage unit and a printing unit.

The foregoing detailed description has set forth a few of the many formsthat the invention can take. It is intended that the foregoing detaileddescription be understood as an illustration of selected forms that theinvention can take and not as a limitation to the definition of theinvention.

1. An adaptive equalizer for high-speed serial data, comprising: aprogrammable equalizer for equalizing an input serial data signal togenerate an equalized serial data signal, wherein the equalization isbased on an optimal equalization mode; a signal quality meter forcomputing an eye width indication based on the equalized serial datasignal, wherein the eye width indication is an indicative of the qualityof the equalized serial data signal, the a signal quality meterincludes: a clock and data recovery (CDR) circuit for recovering theequalized serial data signal using a number of N over-samplers and aN-phase sampling clock; a number of N transition detectors connected tothe number of N over-samplers for detecting rise or fall transitionsoccur at each phase sample; a number of N counters connected to the Ntransition detectors for counting the number of transitions during aprogrammable counting period; a number of N comparators connected to theN counters for comparing the number of transactions counted by eachcounter to a programmable threshold, wherein the comparison resultsdetermine the location of each phase sample relatively to an eye of aneye diagram; a pattern analyzer for generating the eye width indicationby identifying a longest sequence of contiguous phase samples inside aneye of the eye diagram; a timer for timing the programmable countingperiod; and a decision unit for determining the optimal equalizationmode based on the eye width indication.
 2. The adaptive equalizer ofclaim 1, further comprising: a memory for storing a plurality ofequalization modes.
 3. The adaptive equalizer of claim 2, whereindetermining the optimal equalization mode further comprising:iteratively setting the programmable equalizer to each of the pluralityof equalization modes until the best eye width indication isencountered, wherein the equalization mode that results with the besteye indication is the optimal equalization mode.
 4. The adaptiveequalizer of claim 3, wherein equalization modes to which theprogrammable equalizer is set are kept in a sorted list.
 5. The adaptiveequalizer of claim 4, wherein the sorted list is created by:characterizing behavior of the programmable equalizer in a predefinedrange of equalization parameters; and storing the equalization modesassociate with the equalization parameters in a sorted order, if onlyone maximum eye width is detected within the characterization range,wherein the equalization modes are sorted according to progression ofequalization parameters.
 6. The adaptive equalizer of claim 1, whereincounts of each of the counter above the programmable threshold indicatethat a respective phase sample is outside the eye of an eye diagram andcounts below the programmable threshold indicate the phase sample isinside the eye of the eye diagram.
 7. The adaptive equalizer of claim 1,wherein the serial data signal is transmitted at a transfer rate of atleast 3 Giga bit per second (Gpbs) over a physical cable connecting asource device and a sink device.
 8. The adaptive equalizer of claim 7,wherein the connection between the sink device and the source device isthrough high-speed serial interface.
 9. The adaptive equalizer of claim7, wherein the high-speed serial interface includes at least one ofDisplayPort, universal serial bus (USB) 3, high-definition multimediainterface (HDMI), peripheral component interconnect (PCI), PCI expressPCIe, digital interface for video and audio (DiiVA) and PCI generation2.
 10. A method for adaptively equalizing high-speed serial data,comprising: equalizing an input serial data signal to generate anequalized serial data signal, wherein the equalization is based on anoptimal equalization mode; generating an eye width indication based onthe equalized serial data signal, wherein the eye width indication is anindicative of the quality of the equalized serial data signal; anddetermining the optimal equalization mode based on the eye widthindication by iteratively setting the programmable equalizer to each ofa plurality of equalization modes selected from a sorted list, until thebest of eye width indication is encountered, wherein the equalizationmode that results with the best eye indication is the optimalequalization mode, the sorted list that maintains the plurality ofequalization modes is created by characterizing behavior of theprogrammable equalizer in a predefined range of equalization parameters;and storing the equalization modes associate with the equalizationparameters in a sorted order, if only one maximum eye width is detectedwithin the characterization ranqe, wherein the equalization modes aresorted according to progression of equalization parameters.
 11. Themethod of claim 10, wherein generating the eye width indication furthercomprising: recovering the equalized serial data signal using a numberof N over-samplers and a N-phase sampling clock; detecting rise or falltransitions occur at each phase sample of the N phase using N transitiondetectors; counting the number of transitions at each phase sampleduring a programmable counting period using a number of N counters,wherein each counts determine the location of each phase samplerelatively to an eye of an eye diagram; comparing the number oftransactions of each counter of the N counters to a programmablethreshold using a number of N comparators, wherein the comparisonresults determine the location of each phase sample relatively to an eyeof an eye diagram; and generating the eye width indication byidentifying a longest sequence of contiguous phases inside an eye of theeye diagram.
 12. The method of claim 11, wherein each count above theprogrammable threshold indicates that a phase sample is outside the eyeof an eye diagram and each count below the threshold indicates that thephase sample is inside the eye of the eye diagram.
 13. An adaptiveequalizer for high-speed serial data, comprising: a programmableequalizer for equalizing an input serial data signal to generate anequalized serial data signal, wherein the equalization is based on anoptimal equalization mode; a signal quality meter for computing an eyewidth indication based on the equalized serial data signal, wherein theeye width indication is an indicative of the quality of the equalizedserial data signal; and a decision unit for determining the optimalequalization mode based on the eye width indication, the decision unitis further configured to iteratively set the programmable equalizer toeach of a plurality of equalization modes selected from a sorted list,until the best of eye width indication is encountered, wherein theequalization mode that results with the best eye indication is theoptimal equalization mode, wherein the decision unit is furtherconfigured to create the sorted list that maintains the plurality ofequalization modes by characterizing behavior of the programmableequalizer in a predefined range of equalization parameters; and storingthe equalization modes associate with the equalization parameters in asorted order, if only one maximum eye width is detected within thecharacterization range, wherein the equalization modes are sortedaccording to progression of equalization parameters.